This invention relates to electrical power converters and, more particularly, to such a converter for n-winding switched reluctance motors.
Switched reluctance motors (SRM) are well known in the art. Although well known, SRM and their associated drives have not found broad acceptance. One reason for the lack of acceptance is the cost associated with the motor drive. SRM conventionally include a non wound rotor and a stator having a plurality of windings (n-winding) associated with the stator. N-winding switched reluctance motors, whether single phase or poly phase design, typically employ a power converter. In a three-phase motor, for example, the conventional converter requires two terminal connections per motor winding. Further the converter requires at least twelve (4n) solid state devices to provide electrical energy to the winding. Certain of these devices are diodes, a number of which are connected to form a diode bridge circuit; which, in combination with a capacitor also typically used in the converter, yields a relatively low power factor. The capacitor may use a precharge circuit and the diode bridge needs a high surge rating to protect against diode failure.
Besides the above, conventional converters require a high-voltage driver for each winding. The result is a converter, which, while functional, has a cost which is increased not only by the number of components required, but also by the high performance characteristics of those components.
Efforts have already been made to address various of the drawbacks found in conventional converters of the type described above. In U.S. Pat. No. 4,684,867 ('867) for example, there are shown three circuits (see FIGS. 5, 7, and 8) which attempt to simplify a converter's circuitry. In each instance, for example, the number of solid state devices required to provide electrical energy to the windings has been reduced. Further, a capacitor is provided for storing a charge produced by the excess current flowing through each of the motor windings. The capacitor is said to be chargeable to a voltage in excess of the dc source voltage for the circuit. To reduce the charge on this capacitor, each of the three described embodiments includes a bleed circuit for bleeding charge off the capacitor. In two of the embodiments (FIGS. 5 and 7), this bleed circuit is a resonant circuit including an inductor. In the other embodiment (FIG. 8), the bleed circuit includes a resistor. Regardless of the manner in which the bleed circuit is effected, the circuit operates to reduce the charge on the storage capacitor.
While achieving certain advantages over conventional converters, circuits of the type shown in the '867 patent, do have disadvantages. For example, the power factor obtainable from these circuits is no better than that obtainable from conventional converters. In addition, the energy storage requirements of the capacitor are such that the storage capacitor is no smaller than that found in conventional converters. Preferably, the storage capacitor should be significantly smaller yet still produce a constant output torque. Use of a smaller capacitor also permits capacitor construction from less expensive, and more reliable material.
Prior art designs of the type shown in the '867 patent also require a precharging circuit for controlling inrush current charging the storage capacitor whenever power is first applied to the circuit. The invention disclosed hereinafter does not require a precharging circuit.